This fix tries to address the issue raised in 29975 where
it was not possible to specify `attachable` flag for networks
in compose format.
NOTE: Compose format aleady supports `labels` in networks.
This fix adds the support of `attachable` for compose v3.1 format.
Additiona unit tests have been updated and added.
This fix fixes 29975.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>
Yong Tang
Daniel Nephin